CAMP SHORAB, Afghanistan, April 30 (Reuters) - The U.S. Marine Corps has returned to Helmand, the restive province in southern Afghanistan where it fought years of bloody battles with the Taliban, to help train Afghan forces struggling to contain the insurgency.

Many of the 300 Marines coming to Helmand as part of the NATO-led Resolute Support training mission are veterans of previous tours in the province, where almost 1,000 coalition troops, mostly U.S. and British, were killed fighting the Taliban.

When they left in 2014, handing over the sprawling desert base they knew as Camp Leatherneck to the Afghan army, the Marines never expected to return. The fact that they are back underlines the problems Afghan forces have faced since being left to fight alone.

Despite a warning from U.S. Defense Secretary James Mattis last week that 2017 would be a tough year, though, the tone as the deployment began was positive.

"I was excited to come back," said Staff Sergeant George Caldwell, who had previously spent eight months in the far south of Helmand that mixed combat operations with training the Afghan border police.

"I have a lot of time invested in Helmand province. We have many, many years of combat operations and we'd hate to see the region become unstable," he said at the margins of a ceremony marking the Transfer of Authority for the training assignment.

Thousands of Marines served in Helmand over the years between 2009 and 2014 during some of the most intense fighting seen by foreign troops in Afghanistan.

American officers at the ceremony attended by the top U.S. commander in Afghanistan, Gen. John Nicholson promised continuing commitment to helping Afghan forces but the Marines are coming back at a difficult moment.

Their mission this time is not to fight but to train and help Afghan forces, although the strong defensive measures around the base underscore the risk they face in Helmand, one of the heartlands of the Taliban insurgency.

The Afghan army is still reeling from a devastating attack in the northern city of Mazar-i-Sharif this month in which Taliban suicide commandos killed 135 soldiers, according to official figures and double that number by other accounts.

Large stretches of Helmand, source of much of the world's illegal opium supply, are in the hands of the Taliban insurgents, who have steadily pushed back Afghan forces that now control less than 60 percent of the country.

Corruption and poor leadership are still an issue, despite efforts to stamp out problems such as bribery, troops selling weapons and ammunition or non-existent "ghost soldiers" kept on the rolls to allow their pay to be stolen.

In March, a previous commander of the Afghan army 215 Corps in Helmand was arrested, a year after he had been sent to the province root out fraud and corruption in the unit.

"There's some of that," said Captain Zachary Peterson, part of the army-led Taskforce Forge handing over to the Marines of Taskforce Southwest.

"But you can't let one bad apple make you be down on the group as a whole, when the majority of these guys are good people and they want to see good things for their country."

He said Afghan forces had made major improvements in conducting offensive operations against the Taliban, who on Friday announced the start of their annual spring campaign, when warmer weather usually leads to heavier fighting.

"Their attitude and their op tempo right now, all the operations they're doing, are really encouraging," he said.

Some 8,400 American troops are based in Afghanistan as part of Resolute Support as well as a separate counterterrorism mission against Islamic State and Al Qaeda, but Gen. Nicholson said earlier this year a few thousand more would been required to end the "stalemate" with the Taliban.

The Trump administration is currently conducting a review of U.S. policy for Afghanistan, where American troops have now been stationed for more than 15 years.

While most are no longer usually involved in combat operations, the dangers they still face were underlined last week when two army Rangers were killed in the eastern province of Nangarhar fighting Islamic State militants.

Last month, three U.S. soldiers were wounded in Camp Shorab itself, the network of bases of which the Marines' Leatherneck facility was once a part, when an Afghan soldier opened fire on them in a so-called "green on blue" incident.

The base is a dusty expanse of barbed wire fences, checkpoints, huts and blast walls with a faint smell of latrines in the air. Movement is restricted and security remains high, placing an additional strain on the troops.

"This is a small, cramped position," said army Chaplain (Capt.) Sidney Aaron, who helps look after morale and welfare. "My soldiers and these Marines, they go out every day, they have to be on guard, 24/7, so it is a long time." (Reporting by James Mackenzie; Editing by Sam Holmes)